When such seat belts are in use and an automobile crash occurs, the belt tightener such as the inertial-reel, which as a rule is mounted on the floor or on the seat of the automobile, very rapidly pulls against the component by which the buckle is fixed, ordinarily called the stalk, so that the normally loose seat belt is stretched tight. In this tightening process, the individual elements of the buckle assembly, namely the stalk, the locking element and the tongue, are subjected to a high shock loading. However, the buckle assembly must be constructed so that this shock loading does not cause any malfunction; the buckle must remain reliably closed during the shock loading that accompanies the tightening process and during the subsequent loading by the tensile stress imposed on the belt as it restrains the seat occupant. Afterward the buckle must be easy to open.
In tests of buckle function, therefore, a distinction must be made between the high-energy shock loading during the tightening process and the loading during restraint of the occupant.
The present method and associated testing apparatus relate exclusively to the high-energy shock testing.
In the present state of the art of testing the buckles of seat belts equipped with tightening mechanisms, it is characteristic that for every series of tighteners and associated buckles, the reliable locking function of the buckle is tested by triggering the tightener in a statistically sufficient number of units in the series.
In such methods only nominal loads for the particular type of construction are imposed, but no overloading. However, overloading is necessary to determine the actual functional limits of the buckle or to build in and demonstrate the safety factor desired for specific ranges of overloading.
The problem to be solved thus consists of two closely related subproblems:
First, the shock loading must be quantitatively monitored and controlled separately for each component of the functional assembly in order to obtain absolute load values for each component and to measure quantitatively the distribution of shock in the various components according to their structural relationships in the assembly.
Second, reliably reproducible experimental conditions must be ensured for acceleration amplitudes greater by as much as 3- to 4-fold than the loads that actually occur in practice.
The current state of the art of shock production for test objects of the nature and size of seat-belt buckles is characterized by apparatus in which a cable-guided carriage is dropped through a frame, as described in principle in U.S. Pat. No. 3,426,578 and German Gebrauchsmuster DE-GM 69 34 387. Such conventional drop-frame apparatus provides for shocks of up to ca. 10,000 g to be imposed over times in the range of .mu.s, depending on the drop distance, the mass and shape of the drop carriage, the material and the impact medium. Alternatively, for lower shock accelerations, an oscillating-hammer test stand can be used, such as is described in Japanese Patent Specification JP 63-11 33 42 A. A sensor for shock measurement is mentioned in Japanese Patent JP 63-28 44 51 A, but it is not attached to the drop carriage or to the specimen to be tested, but rather to a neutral intermediate element.
When the peak acceleration is increased by 3- to 4-fold, however, the drop carriage itself can be set into oscillation, which interferes with the measurement and makes it impossible to ensure reproducibility of the shock, so that the range of application is limited. Furthermore, after a few shocks the impact surface of the carriage becomes deformed and develops cracks, which makes the carriage unsuitable for further use. The same applies to the impact medium, onto which the carriage falls.
Constructional means of reducing the intrinsic oscillations of the drop carriage and indications of the shape and selection of material of the drop carriage for the load values required here are not disclosed in the literature of the field.
Thus, the object of the present invention is to provide a method and an associated testing apparatus for a functional assembly that enables rapid, simple and relatively inexpensive reproduction of both the shock loads typically encountered in operation of the assembly and overloads for the applications described above in a manner wherein the shock loads are imposed in a quantitative and reliably reproducible manner.